Up to now, a technique for detecting an actual combustion state of an internal combustion engine with the use of an in-cylinder pressure sensor, and subjecting the detected combustion state to various corrections, to thereby control a combustion engine has been known. As disclosed in Patent Literature 1, a technique for performing an external exhaust gas recirculation for introducing a relatively large amount of exhaust gas into a cylinder for the purpose of reducing the amount of nitrogen oxide (NOx) to be generated in a diesel engine has been known.
In the diesel engine, it is allegedly difficult to achieve all of a high thermal efficiency, a low fuel consumption, a reduction in the amount of NOx, a reduction in the amount of hydrocarbon (HC), the suppression of vibration noise, and the limitation of an maximum in-cylinder pressure. In particular, it is difficult to achieve both of the high thermal efficiency and the reduction in the amount of NOx. In the configuration of Patent Literature 1, the maximum in-cylinder pressure or a maximum change rate of the in-cylinder pressure is detected, and a fuel injection timing and the amount of exhaust gas (EGR amount) are corrected to obtain a desired maximum in-cylinder pressure and a desired maximum change rate of the in-cylinder pressure, thereby being capable of reducing the amount of NOx.
As described above, in the case of using the external exhaust gas recirculation, because a low temperature combustion is generated, the maximum in-cylinder pressure can be controlled to be reduced, and the amount of NOx is reduced. However, a specific heat ratio of the cylinder is lowered by impurities contained in the exhaust gas, and the thermal efficiency is lowered. In this way, the reduction in the amount of NOx has a contradictory relationship with the high thermal efficiency.
Up to now, a fuel injection device of a twin needle type which adjusts the amount of lift of an outer needle and an inner needle coaxially housed in a housing to perform the injection control of a fuel has been known.
On the other hand, a diesel cycle and a Sabathe cycle having a constant pressure heating process are thermal cycles used in a compression ignition engine such as a medium-speed or high-speed diesel engine, and have excellent thermal efficiency. For that reason, the diesel cycle and the Sabathe cycle can take out a large amount of energy with a small amount of fuel, and therefore are small in carbon dioxide emission and environmentally friendly. Therefore, in order to create a favorable combustion state due to the diesel cycle and the Sabathe cycle, it is necessary to control the fuel injection amount with high precision.
In a fuel combustion device of a twin needle type in the configuration of Patent Literature 2, pressures on a back side of an inner needle and an outer needle are adjusted to control the fuel injection amount. For that reason, the fuel injection amount can be changed in two stages of the inner needle and the outer needle. However, because the fuel injection amount of the fuel injection device thus configured is determined according to a minimum flow channel area of a first injection hole and a minimum flow channel area of a second injection hole, the fuel injection amount is converged to a constant amount. When a certain amount of fuel injection amount is given, combustion is not promoted during a constant pressure heating process of the diesel cycle and the Sabathe cycle. This causes a problem that when a piston in a combustion chamber of an engine is retracted, in other words, when a volume in the combustion chamber expands, the pressure decreases in spite of the constant pressure heating process, and an excellent combustion state is not obtained.
In the diesel engine, because the fuel injected from a fuel injector is self-injected due to a compression in the combustion chamber, a compression ratio is high as compared with a gasoline engine, and a peak of the in-cylinder pressure in the combustion chamber, which is generated by the combustion of fuel, is increased. When the peak of the in-cylinder pressure (hereinafter referred to as “maximum in-cylinder pressure Pmax”) exceeds an allowable upper limit pressure, there is a concern that the reliability of the engine is lowered. For that reason, it is conceivable that the strength of the engine is improved to increase the allowable upper limit pressure “Pulim”. However, when the strength of the engine is improved, there is a concern that the weight and cost of the engine are increased.
Under the circumstances, as disclosed in Patent Literature 3, in an operation region on a high load side where a maximum in-cylinder pressure Pmax is increased, a fuel injection control for executing a rising injection pattern in which a fuel injection rate is gradually increased is performed. As a result, the in-cylinder pressure can be gradually increased according to an expansion of a combustion chamber volume in an expansion stroke, and the maximum in-cylinder pressure Pmax can be reduced.
However, when the fuel injection rate is gradually increased as disclosed in Patent Literature 3, a long time is required until the in-cylinder pressure reaches the maximum in-cylinder pressure Pmax, and the thermal efficiency is lowered. As a result, a power of the diesel engine is also decreased.
[Patent Literature 1] JP-2010-174737-A
[Patent Literature 2] JP-2009-062920-A
[Patent Literature 3] JP-2009-5117-A